1. Field of the Invention
The present invention relates to a device for detecting a rotation rate that detects a rotation rate of a rotor which applies pressure to a fluid, in the path of the fluid, due to the rotation of the rotor.
2. Description of the Related Art
Conventionally, a device for detecting a rotation rate is known that detects a rotation rate of a turbine of a turbocharger which applies pressure to air to compress it and to force the compressed air into a combustion chamber.
For example, according to a device for detecting a rotation rate disclosed in Japanese Patent Application Unexamined Publication No. 2003-240788, a sensor detects a pressure or a sound generated by the rotation of a rotor in order to detect the rotation rate of the rotor. Further, as shown in FIG. 4A and FIG. 4B, a device for detecting a rotation rate uses an eddy current sensor 110 to detect a rotation rate of a turbine. A plurality of blades 104 are formed on the turbine in the radius direction.
The eddy current sensor 110 shown in FIG. 4A and FIG. 4B is disposed on a housing member 100 forming the path of the fluid, so as to face the blades 104 of the turbine. The eddy current sensor 110 has a coil 112 that generates a magnetic field based on an alternating excitation current. When the blades 104 pass through the magnetic field generated by the coil 112, an eddy current is generated in the blades 104 to offset the magnetic field generated by the coil 112. The intensity of the magnetic field changes due to the eddy current generated in the blades 104, thereby changing the current that flows through the coil 112. As a result, the eddy current sensor 110 detects that the blades 104 pass through the magnetic field. The device for detecting a rotation rate detects a rotation rate of the turbine by detecting a change in the current that passes through the coil 112.
When a high-frequency alternating excitation current is supplied to the coil 112, an eddy current is generated in the housing member 100 around the eddy current sensor 110 due to a factor other than the rotation of the blades 104. When the eddy current is generated in the housing member 100, the eddy current sensor 110 cannot determine whether the current generated in the coil 112, due to a change in the intensity of the magnetic field, changes because of the eddy current generated in the housing member or because of the blades 104 that pass through the magnetic field. When the intensity of the magnetic field generated by the coil 112 decreases due to the eddy current generated in the housing member 100, the eddy current generated in the blades 104 becomes small when the blades 104 pass through the magnetic field. At the same time, when the blades 104 pass through the magnetic field, the current generated in the coil 112 also becomes small. As a result, it becomes difficult to detect the blades 104 that pass through the magnetic field, and there is a possibility that the device for detecting a rotation rate cannot detect the rotation rate of the turbine.
Accordingly, as shown in FIG. 4A and FIG. 4B, when a concave portion 102 is formed in a ring shape on the housing member 100 to cover the surrounding of the eddy current sensor 110, the concave portion 102 can prevent an eddy current from being generated in the housing member 100 around the eddy current sensor 110.
However, when the ring-shaped concave portion 102 is formed around the eddy current sensor 110 as shown in FIG. 4A and FIG. 4B, a part of air that is about to be compressed by the rotation of the blades 104 passes through the concave portion 102 and flows downstream, instead of passing through between the blades 104 and the surface of the path of the fluid formed on the housing member 100. Consequently, the compression efficiency of the blades 104 decreases.